One of the major processing problems encountered in optically correlating synthetic aperture radar data from spacecraft radars is accurately determining the attitude of the spacecraft with respect to the rotating earth. Orbital parameters are necessary in order to reduce synthetic aperture radar data on the ground; and without accurate knowledge of these parameters ambiguities are introduced in the final imaging process. These ambiguities appear in the form of double or overlapping image or data gaps.
For a given optical correlation process required to produce target imagery of synthetic aperture radar, optical parameters of the correlator elements, that is, aperture size, focal lengths, etc., are directly related to the orbital parameters of the synthetic aperture radar instrument. These parameters are fixed and based on predetermined orbital parameters (predicts) of the spacecraft, and are only as accurate as the assumptions made of the spacecraft's path and the synthetic aperture radar instrument's boresight coordinates with respect to a rotating earth. Occasionally these predicts are inaccurate, and most often are unavailable during actual correlation of the synthetic aperture radar data. The synthetic aperture radar instrument boresight coordinates, when inaccurate, produce inaccurate correlator element settings which result in image ambiguities. Most often ambiguities that are introduced are due to the spacecraft's attitude control deviations. These ambiguities are introduced into the ultimate synthetic aperture radar image by a shift in the doppler/spectrum due to a spacecraft attitude change. When a doppler shift occurs, the optical correlator operator generally stops the imaging process and repositions several optical elements, thereby recentering the spectrum of the output from a Fourier transform filter in order to avoid target image ambiguities. This manual repositioning significantly slows the optical correlation process and decreases the throughput rate of an optical correlator. The present invention solves this problem by providing a closed loop correction system for detecting and automatically altering at least one of the optical elements to compensate for doppler shift, thereby eliminating a need for the previously described manual correction.